john deere g running odd

[Trey Parker]

We just got g together with a 7 3/4 bore and 9 3/4 stroke. It runs great at idle but when its at full throttle it starts missing really bad. We have changed carburetors and plugs and its still doing it. Does anyone know what's going on? Thanks.

 

[Husker44A]

Good plugs, wires? I would check timing. Mag or a dist?

 

[Trey Parker]

Distributor

 

[Husker44A]

Also check the points for pitting. cap,rotor for black streaks from the fire jumping and coil too.

 

[thesmoothedeere]

How long do your plugs last ? I started up my tractor yesterday after sitting 6 months . It has the same plugs for 5 years now . I got hestitation and some missing when it gets full throttle . I figure my plugs need changed , haven't even looked at them yet but i'll put new ones in .

 

[730-LP]

Did you use new valve springs?jf not the valves may be sucking open before they are suppose to open.Also mark the flywheel and take a timing light and see if the distributot is advancing

 

[Mike Aylward]

Are you still running the gravity feed on your fuel system? It may be that you need to install an electric fuel pump and regulator to keep up with the fuel needs of that many cubic inches and possibly enlarge other areas of the system as well (inlet valve, lines, etc.). Each cylinder has more cubic inches than both did originally. With the tremendous vacuum of that many inches you may simply be starving for fuel. Mike

 

[just thinking]

You just spent a pile of money. I would call the engine builder and pick his brain.

 

[Mike N Mo]

Here's a couple more things to stir around. 1st, what, if any, ignition upgrades have you done? I'll assume that the engine is somewhere in the 9-10:1 mechanical compression ratio. As combustion chamber pressure increases, the voltage required to jump the plug gap increases. This would be more likely to come into play under a load than no load high idle, but still something to think about.
2nd, what are the cam specs, particularly duration and overlap. Is it timed to the cam grinder's spec?

Let us know if you come up with anything. Mike

 

[Mike N Mo]

Follow-up to my earlier post. Is this a 1 barrel or a 2 barrel carb? If it's a 2 barrel, as Mike Aylward suggested, you'll need a fuel pump. The needle/seat is smaller than a 1 barrel and will not support the fuel needs of a big cube engine without a pump. You'll also need to drill out the jets, somewhere around a # 49-50 to start with, possibly larger than that. Mike

 

[Trey Parker]

Needs new valve springs. Thanks for the help everyone

 

[Trey Parker]

Needs new valve springs. Thanks for the help everyone